The present invention relates to an apparatus and a method for manufacturing an optical information recording medium obtained by bonding two substrates.
The technology of reproducing or recording information with high density using a laser beam is well known, and has put an optical disk to practical use. Optical disks can be classified into reproduce-only-type disks, additive-write-type disks, and rewrite-type disks.
Reproduce-only-type disks are manufactured as compact disks recording music information, laser disks recording image information, and so on. Additive-write-type disks are manufactured as document files, static image files, and so on. Rewrite-type disks are manufactured as data files for personal computers or the like.
A typical optical disk is manufactured by covering approximately 1.2 mm thick transparent resin circular substrate one side of which is an information layer with a protective coating film or the like, or with a protective plate having the same size as the substrate bonded on the information layer.
Recently, it has been investigated to use a shorter laser waveform and an objective having a larger numerical aperture (NA) in order to realize a high-density optical disk.
However, an optical disk with a shorter waveform and a larger NA has a problem that an angle between the inclination of a disk and the direction of an input laser beam, that is, a tilt allowance, is small.
It is known, in this case, that a thinner circular substrate (hereinafter referred to simply as a substrate) is effective to have a larger tilt allowance. For example, a substrate of a digital video disk (DVD) is 0.6 mm thick. Since a 0.6 mm thick resin substrate is not mechanically strong, two substrates are bonded for reinforcement with the information recording surface set inside.
As a method of bonding two substrates, there is a radiation hardening method in which radiation-hardening resin is applied to one substrate on which another substrate is bonded with radiation emitted for hardening. In this connection ultraviolet rays (UV) are used as radiation.
In this radiation hardening method, the radiation-hardening resin is circularly applied to a substrate rotating at a low speed, then another substrate is bonded on it into one sheet. After the radiation-hardening resin is sufficiently spread between the substrates while rotating the substrates at a high speed, the radiation is emitted for a hardening process.
When the radiation hardening process is performed, the substrate is attached at the end of a rotation arm for rotary movement of the emission area of a radiation lamp.
In this rotary movement system, the center of the substrate conventionally supported by the rotation arm is set as the center of a light source, for example, the center of the longitudinal length when a cylindrical lamp is used. When two lamps are used, the lamps are arranged in parallel with the center of the substrate passing through each of the centers of the light sources.
In the optical recording medium manufacturing apparatus with the above mentioned configurations, a substrate rotates such that the center of the substrate passes through the center of the light source of a cylindrical lamp. Therefore, the rotating speed at an outer circular portion of the substrate is high while the rotating speed at an inner circular portion of the substrate is low with the center of the light source of a cylindrical lamp set as the center. As a result, there arises the problem that the amount of accumulated radiation at the outer circular portion is smaller than that at the inner circular portion.
Therefore, the accumulated radiation cannot be symmetrically applied onto a substrate, thereby causing poor resin hardening balance and uneven surface of a bonded substrate.
The present invention has been developed to solve the above mentioned problem and aims at improving the bonding precision without uneven disk surface.
To attain the above mentioned subject, an object of the present invention is to remove the difference in the amount of accumulated radiation between an outer circular portion and an inner circular portion of a disk by positioning the center of the light source of a lamp at a position radially outside the rotation locus of the center of the disk supported by the rotation arm.
Therefore, regardless of the rotating speed, the amount of accumulated radiation can be equalized between the radially inner and outer portions.
The optical recording medium manufacturing apparatus according to the present invention includes: a lamp for emitting radiation to two sheets of disks attached to each other with a radiation-hardening resin layer sandwiched therebetween; and a rotation arm for supporting the two disks and rotating them such that the disks can traverse an emission area of the lamp. The center of the light source of the lamp is positioned radially outside the rotation locus of the center of the disks, the rotation locus being drawn while the disks are supported by the rotation arm.
With the above mentioned configuration, the distribution of hardened resin can be symmetric, and the amount of accumulated radiation can be substantially equalized between the inner and outer circular portions on a disk, thereby manufacturing a precision optical recording medium without any uneven surface.
In addition, in the optical recording medium manufacturing apparatus, the lamp is cylindrical, and the longitudinal axis of the cylindrical lamp traverses the rotation locus of a disk.
With this configuration, the amount of accumulated radiation can be equalized with respect to a point light source.
Also with the above mentioned configuration, the longitudinal axis of a cylindrical lamp is arranged to be radially outside the center of the rotation arm.
In this case, since a substrate traverses the lamp at right angles along the circumference of the disk, the amount of radiation can be further equalized.
A method for manufacturing an optical recording medium according to the present invention is a method in which two disks attached to each other with a resin sandwiched therebetween are rotated around a perpendicular axis outside the disks while one or more lamps emit radiation from above a part of a rotation locus area of the disks, thereby to manufacture an optical recording medium. At this time, an amount of radiation per unit area at each point on the disks is proportioned to a distance of said each point from the perpendicular rotation axis.
As described above, according to the present invention, the amount of the accumulated radiation can be well balanced on a hardened resin substrate although it is rotated by a rotation arm, thereby manufacturing an optical recording medium free of uneven surface with high bonding precision.